Pressured-water nuclear reactors include a vessel containing the reactor core which is submerged in the pressurized water for cooling the reactor.
The reactor vessel, of generally cylindrical shape, includes a hemispherical head which may be attached to its upper portion. The head is pierced with openings in the region of each of which is fixed, by welding, in a vertical position, a tubular penetration part constituting an adaptor providing for the passage of a follower of a rod for controlling the reactivity of the core or of a means of measurement inside the core, such as a thermocouple column.
Each of the tubular penetration parts is engaged in an opening penetrating the vessel head and is welded onto the concave lower surface of the head, by an annular weld.
The adaptor is welded onto the vessel head by an orbital welding technique, using a GTAW welding head which includes an electrode and means for feeding a metal filler wire into the region of the electrode. The welding head is moved rotationally about the axis of the penetration and enables the filler metal to be laid down in the form of successive weld beads, inside an annular bevel which surrounds the penetration part and is made, at least partially, inside the head in the region of the penetration opening.
In general, the annulus-shaped welding bevel, which has as axis the axis of the penetration, has a meridional cross-section which widens out from the bottom of the bevel inside the head as far as the outer end of the bevel in the region of the concave surface of the head.
The width of the bevel, even at its narrowest portion, is generally greater than the width of a weld bead made of filler metal which may be laid down by the welding head, during a rotation about the axis of the penetration.
It is therefore necessary to fill in the bevel by juxtaposing, according to the width of the bevel, weld beads which are formed during successive passes.
The weld beads, juxtaposed according to the width of the bevel, constitute a layer which is formed in a plurality of successive passes during each of which the welding head carries out one complete revolution in rotation about the axis of the penetration.
The successive layers of filler metal, constituted by juxtaposed weld beads, are themselves superposed, so as to fill in the entire bevel. The welding of an adaptor, which requires numerous successive orbital-welding passes, is therefore generally a lengthy operation during which a significant volume of filler metal is laid down.
Furthermore, the position of the electrodes and, in particular, their orientation inside the bevel, must be adjusted during each of the successive passes in order to obtain optimum welding conditions. At the same time, the filler wire must be fed into the weld zone with an orientation and in a position which are both variable.
Because of the significant mass of filler metal laid down, large deformations may occur in the region of the penetration.
Furthermore, in the case of a spherical vessel head, the welding is performed over the lower concave surface of the head, so that the adaptors, which are distributed over the spherical-cap-shaped surface of the head and the axis of which has a constant direction which is the vertical direction, have bevels for connecting to the curved surface of the head, the shape of which is complex.
Because it is also necessary to perform at least some welding passes in positions of electrodes which are inclined in relation to the axis of the penetration, the adjustment of the position and of the inclination of the electrode may turn out to be extremely complicated, if not impossible.
In order to overcome these drawbacks, a new type of orbital GTAW welding, generally designated as a narrow-bevel welding method, has been developed. According to this method, the width of the bevel is reduced to a minimum so as to permit the bevel to be filled in by superposed layers each constituted by a single weld bead formed during a single pass.
As a result, the volume of metal laid down is significantly reduced, as are the deformations and residual stresses in the region of the weld zone. Furthermore, the welding time is significantly shorter, which enables the time necessary for manufacturing the vessel head to be significantly decreased.
Another major advantage of the narrow-bevel welding method is that the electrode has a substantially constant inclination from one pass to another, the electrode being able to be placed in a direction substantially parallel to the axis of the penetration.
However, the narrow-bevel welding method is not particularly suited to the case of welding adaptors at any point on the surface of a spherical-cap-shaped vessel head, even if the adjustment of the position and of the inclination of the orbital welding electrode is much easier than in the case of conventional wide-bevel welding.
In fact the surface of the bottom of the bevel, or of the successive layers laid down on the bottom of the bevel, has a complicated geometrical shape for all the adaptors, with the exception of the adaptor located at the central portion of the head, at the top of the spherical cap.
The bottoms of the bevels, or the successive welding layers, have the shape of skewed surfaces which may be considerably inclined in relation to a horizontal plane.
The usual kinetics for moving the orbital-welding electrodes do not assure satisfactory welding conditions, i.e., a constant distance between the end of the electrode and the bottom of the bevel to be covered and a speed of welding, by laying down filler metal, which is constant.
This difficulty occurs not only in the case of adaptors for penetrating a vessel head but also in the case of orbital welding of any cylindrical part onto a curved wall in the region of an opening penetrating the wall, when the axis of the penetration does not pass through the center of curvature of the wall.